Cerium is a rare earth element of the lanthanide series. The oxide form (CeO2) has routinely been used in polishing glass, but current research is focused on use of cerium oxide nanoparticles in catalytic converters for automobile exhaust systems, oxygen sensors, an electrolyte for solid oxide fuel cells or as an ultraviolet absorbent. While most of the rare earths exist in a trivalent state (+3), cerium also occurs in a tetravalent (+4) state and may flip-flop between the two in a redox reaction. It is established that cerium oxides make excellent oxygen buffers, because of this redox capacity. As a result of alterations in cerium oxidation state, cerium oxide forms oxygen vacancies or defects in the crystal lattice structure by loss of oxygen and/or its electrons. The valence and defect structure of cerium oxide is dynamic and may change spontaneously or in response to physical parameters such as temperature, presence of other ions, and partial pressure of oxygen.
Studies have shown that with decrease in particle size, cerium oxide nanoparticles show formation of more oxygen vacancies within their crystal lattice. We hypothesized that the increased surface area to volume in nanoparticles enables CeO2 to regenerate its activity and thereby act catalytically. In the case of transition metal oxides, a thorough analysis of vacancies has lead to the understanding of the fundamental nature of the catalytic reactivity. However, this knowledge has been lacking with respect to the rare earth oxides. In addition, there has been no reported literature elucidating the molecular mechanism of any catalytic activity of these nanoparticles in biological systems.
Cerium oxide nanoparticles have a unique electron structure that is similar to chemical spin traps such as nitrosone compounds and mixed valence state ceria nanoparticles have been recently shown to apparently act as biological antioxidants. It has been proposed that this antioxidant activity is mediated at oxygen vacancies at the surface of the nanoparticle. If so, then one cerium oxide nanoparticle may offer many sites for catalysis, whereas pharmacological agents or enzymes offer only one active site per molecule. In addition, the electron defects in ceria nanoparticles may not be destroyed after their initial reaction with reactive oxygen species and thus these nanoparticles may be potent catalysts in a living cell. Although there is growing evidence that ceria nanoparticles impart protection to living cells, the molecular mechanism of the antioxidant properties of cerium oxide nanoparticles has yet to be elucidated. Previous studies have suggested, based on observations of the impact of ceria nanoparticles on cultured cells, that ceria nanoparticles can act as radical scavengers and redox cycling antioxidants.